When towing It spiked up to almost 160° on a long ~4% slope 90° ambient temperature.
If that reading is in °F, temperature is great. But I highly doubt it.
Transmission temperature point formulas change between generations and the readers can get it wrong if they don't have the latest updates. For example, in my RAV4, the formula changed in the middle of a generation, 2009 and 2010 use different formulas. Techstream software is supposed to be updated with the readings, especially for newer cars.
Most of the transmissions share cooling space with the engine cooling fluid and hence they get thermally at equilibrium, at around 185-190°F. An auxiliary cooler helps dropping that about 10-15°F, but only at speed, due to air flow. In stop-and-go traffic, the aux cooler is not so helpful and temperatures tend to go up. Using the AC in the car turn on the main fans at low speed, so that sometimes helps in that situation.
As a note, 175°F should be great for transmissions.
Your fluid is highly oxidized so I would recommend an immediate exchange of all of the ATF.
WS is known to get black super fast.
In general that's how the factory Toyota WS fluids behave. They oxidize very fast and Toyota says even not to check the level (so you don't add more oxygen in). In my RAV4, at 70k miles, it was a very dark, violet tint. But at least didn't
smell burned, no towing previously. To me it still doesn't inspire trust. After all it's a fairly new introduced fluid. And the factory fill is not even synthetic.
I drained it multiple times with Valvoline MAxLife and final fill was with AISIN WS (Rockauto has it), because that's a full synthetic fluid.
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